Deposit formation is a persistent problem in a variety of industrial processes involving fluids, such as pulp bleaching, sugar production and filtration. The deposits may remain suspended in the fluid or accumulate on the surface of any material that contacts the fluid. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria.
A primary detrimental effect associated with such deposits is the reduction of the capacity or bore of receptacles and conduits employed to store and convey the fluid. For example, in the case of conduits used to convey scale-contaminated water, the impedance of flow resulting from deposition is an obvious consequence.
However, a number of equally consequential problems arise from utilization of deposit-contaminated fluid. For example, deposits on the surfaces of storage vessels and conveying lines for process water may break loose and become entrained in and conveyed by the process water to damage and clog equipment through which the water is passed, e.g., tubes, valves, filters and screens. In addition, these deposits may appear in, and detract from, the final product derived from the process, such as paper formed from an aqueous suspension of pulp.
Furthermore, when deposit-contaminated fluid is involved in a heat exchange process, as either the “hot” or “cold” medium, scale will be formed upon the heat exchange surfaces contacted by the fluid. Such scale formation forms an insulating or thermal opacifying barrier that impairs heat transfer efficiency as well as impeding flow through the system. Thus, deposit formation is an expensive problem in many industrial fluid systems, causing delay and expense resulting from shutdowns for cleaning and removal of the deposits.
Accordingly, there is an ongoing need for the development of new agents that prevent or inhibit the formation of deposits in fluids and for convenient methods of measuring the effectiveness of these inhibitors. In addition, as natural inhibitors may already be present in the fluids of interest, there is a need for effective methods of characterizing the tendency of industrial and biological fluids as such to form deposits.
For example, the effectiveness of scale inhibitors is manifested by their ability to suppress crystal growth through blocking active sites of potential centers of crystallization and preventing the agglomeration of growing crystals.
Common to the above processes is that they occur at the solid-fluid interface. Therefore, the in situ measurement of the deposition rate in the presence of scale inhibitors at the solid-fluid interface is of particular interest. Traditional measurements mostly relate to the change of the bulk properties of a test fluid such as solubility, conductivity, turbidity and the like following deposit formation. There exist quite a few methods for measuring deposit growth rate, however, fewer methods exist for conducting the measurements in situ at the solid-liquid interface.
Methods for measuring deposit growth rate at the solid-fluid interface that utilize a piezoelectric microbalance are disclosed in U.S. Pat. Nos. 5,201,215, and 6,250,140 and European Patent Application No. 676 637 A1. The principle of piezoelectric mass measurement is based upon the property of a quartz resonator to change its mechanical resonance frequency f0 proportionally to the mass and viscoelastic properties of the deposited material. The change in frequency is expressed as follows:       Δ    ⁢                   ⁢    f    ≈      -                            2          ⁢                      f            0            2                                                N            ⁢                          (                                                μ                  μ                                ⁢                                  ρ                  q                                            )                                ⁢                      1            2                              ⁡              [                              ρ            s                    +                                    (                              ρη                                  4                  ⁢                  π                  ⁢                                                                           ⁢                                      f                    0                                                              )                                      1              2                                      ]            where f0 is the unperturbed resonant frequency of the quartz crystal; N is the harmonic number; μμis the quartz shear stiffness, ρs is the density of quartz; ρS is the surface mass density of the deposit (mass/area), ρ is the density of the medium contacting the resonator and η is the viscosity of the medium contacting the resonator.
Where the viscoelastic properties of the system are negligible or remain constant through the measurements, the surface mass density can be measured using a simplified expression that can be used for the loading causing the resonant frequency change up to 5% (approx. 4.5 mg/cm2):ρs=−C Δf0where C is determined by calibration and is typically equal 1.77×10−5 mg/(sec cm2 Hz) for a 5 MHz quartz crystal.
A method of measuring the rate of calcium carbonate scale onto the surface of the working electrode of a quartz crystal microbalance mounted in an impinging jet cell, where the working electrode is polarized using a potentiostat is disclosed in Gabrielli et al., “Quartz Crystal Microbalance Investigation of Electrochemical Calcium Carbonate Scaling”, J. Electrochem. Soc., 145, 2386-2396 (1998). This method is, however, specific to calcium carbonate scale and is impractical for use in certain circumstances as a constant recirculating flow of test fluid from a reservoir through the impinging jet cell is required. Consequently, a need still exists for more flexible methods of measuring the deposit forming capacity of fluids . . .